Glycolysis

Importance of Glycolysis

Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism.

Learning Objectives

Explain the importance of glycolysis to cells

Key Takeaways

Key Points

  • Glycolysis is present in nearly all living organisms.
  • Glucose is the source of almost all energy used by cells.
  • Overall, glycolysis produces two pyruvate molecules, a net gain of two ATP molecules, and two NADH molecules.

Key Terms

  • glycolysis: the cellular metabolic pathway of the simple sugar glucose to yield pyruvic acid and ATP as an energy source
  • heterotroph: an organism that requires an external supply of energy in the form of food, as it cannot synthesize its own

Nearly all of the energy used by living cells comes to them from the energy in the bonds of the sugar glucose. Glucose enters heterotrophic cells in two ways. One method is through secondary active transport in which the transport takes place against the glucose concentration gradient. The other mechanism uses a group of integral proteins called GLUT proteins, also known as glucose transporter proteins. These transporters assist in the facilitated diffusion of glucose. Glycolysis is the first pathway used in the breakdown of glucose to extract energy. It takes place in the cytoplasm of both prokaryotic and eukaryotic cells. It was probably one of the earliest metabolic pathways to evolve since it is used by nearly all of the organisms on earth. The process does not use oxygen and is, therefore, anaerobic.

Glycolysis is the first of the main metabolic pathways of cellular respiration to produce energy in the form of ATP. Through two distinct phases, the six-carbon ring of glucose is cleaved into two three-carbon sugars of pyruvate through a series of enzymatic reactions. The first phase of glycolysis requires energy, while the second phase completes the conversion to pyruvate and produces ATP and NADH for the cell to use for energy. Overall, the process of glycolysis produces a net gain of two pyruvate molecules, two ATP molecules, and two NADH molecules for the cell to use for energy. Following the conversion of glucose to pyruvate, the glycolytic pathway is linked to the Krebs Cycle, where further ATP will be produced for the cell’s energy needs.

image

Cellular Respiration: Glycolysis is the first pathway of cellular respiration that oxidizes glucose molecules. It is followed by the Krebs cycle and oxidative phosphorylation to produce ATP.

The Energy-Requiring Steps of Glycolysis

In the first half of glycolysis, energy in the form of two ATP molecules is required to transform glucose into two three-carbon molecules.

Learning Objectives

Outline the energy-requiring steps of glycolysis

Key Takeaways

Key Points

  • ATP molecules donate high energy phosphate groups during the two phosphorylation steps, step 1 with hexokinase and step 3 with phosphofructokinase, in the first half of glycolysis.
  • In steps 2 and 5, isomerases convert molecules into their isomers to allow glucose to be split eventually into two molecules of glyceraldehyde-3-phosphate, which continues into the second half of glycolysis.
  • The enzyme aldolase in step 4 of glycolysis cleaves the six-carbon sugar 1,6-bisphosphate into two three-carbon sugar isomers, dihydroxyacetone-phosphate and glyceraldehyde-3-phosphate.

Key Terms

  • glucose: a simple monosaccharide (sugar) with a molecular formula of C6H12O6; it is a principal source of energy for cellular metabolism
  • adenosine triphosphate: a multifunctional nucleoside triphosphate used in cells as a coenzyme, often called the “molecular unit of energy currency” in intracellular energy transfer

First Half of Glycolysis (Energy-Requiring Steps)

In the first half of glycolysis, two adenosine triphosphate (ATP) molecules are used in the phosphorylation of glucose, which is then split into two three-carbon molecules as described in the following steps.

image

The first half of glycolysis: investment: The first half of glycolysis uses two ATP molecules in the phosphorylation of glucose, which is then split into two three-carbon molecules.

Step 1. The first step in glycolysis is catalyzed by hexokinase, an enzyme with broad specificity that catalyzes the phosphorylation of six-carbon sugars. Hexokinase phosphorylates glucose using ATP as the source of the phosphate, producing glucose-6-phosphate, a more reactive form of glucose. This reaction prevents the phosphorylated glucose molecule from continuing to interact with the GLUT proteins. It can no longer leave the cell because the negatively-charged phosphate will not allow it to cross the hydrophobic interior of the plasma membrane.

Step 2. In the second step of glycolysis, an isomerase converts glucose-6-phosphate into one of its isomers, fructose-6-phosphate. An enzyme that catalyzes the conversion of a molecule into one of its isomers is an isomerase. (This change from phosphoglucose to phosphofructose allows the eventual split of the sugar into two three-carbon molecules).

Step 3. The third step is the phosphorylation of fructose-6-phosphate, catalyzed by the enzyme phosphofructokinase. A second ATP molecule donates a high-energy phosphate to fructose-6-phosphate, producing fructose-1,6-bisphosphate. In this pathway, phosphofructokinase is a rate-limiting enzyme. It is active when the concentration of ADP is high; it is less active when ADP levels are low and the concentration of ATP is high. Thus, if there is “sufficient” ATP in the system, the pathway slows down. This is a type of end-product inhibition, since ATP is the end product of glucose catabolism.

Step 4. The newly-added high-energy phosphates further destabilize fructose-1,6-bisphosphate. The fourth step in glycolysis employs an enzyme, aldolase, to cleave 1,6-bisphosphate into two three-carbon isomers: dihydroxyacetone-phosphate and glyceraldehyde-3-phosphate.

Step 5. In the fifth step, an isomerase transforms the dihydroxyacetone-phosphate into its isomer, glyceraldehyde-3-phosphate. Thus, the pathway will continue with two molecules of a single isomer. At this point in the pathway, there is a net investment of energy from two ATP molecules in the breakdown of one glucose molecule.

The Energy-Releasing Steps of Glycolysis

In the second half of glycolysis, energy is released in the form of 4 ATP molecules and 2 NADH molecules.

Learning Objectives

Outline the energy-releasing stepsĀ of glycolysis

Key Takeaways

Key Points

  • The net energy release in glycolysis is a result of two molecules of glyceraldehyde-3- phosphate entering the second half of glycolysis where they are converted to pyruvic acid.
  • Substrate -level phosphorylation, where a substrate of glycolysis donates a phosphate to ADP, occurs in two steps of the second-half of glycolysis to produce ATP.
  • The availability of NAD+ is a limiting factor for the steps of glycolysis; when it is unavailable, the second half of glycolysis slows or shuts down.

Key Terms

  • NADH: nicotinamide adenine dinucleotide (NAD) carrying two electrons and bonded with a hydrogen (H) ion; the reduced form of NAD

Second Half of Glycolysis (Energy-Releasing Steps)

So far, glycolysis has cost the cell two ATP molecules and produced two small, three-carbon sugar molecules. Both of these molecules will proceed through the second half of the pathway where sufficient energy will be extracted to pay back the two ATP molecules used as an initial investment while also producing a profit for the cell of two additional ATP molecules and two even higher-energy NADH molecules.

image

The second half of glycolysis: return on investment: The second half of glycolysis involves phosphorylation without ATP investment (step 6) and produces two NADH and four ATP molecules per glucose.

Step 6. The sixth step in glycolysis oxidizes the sugar (glyceraldehyde-3-phosphate), extracting high-energy electrons, which are picked up by the electron carrier NAD+, producing NADH. The sugar is then phosphorylated by the addition of a second phosphate group, producing 1,3-bisphosphoglycerate. Note that the second phosphate group does not require another ATP molecule.

Here, again, there is a potential limiting factor for this pathway. The continuation of the reaction depends upon the availability of the oxidized form of the electron carrier NAD+. Thus, NADH must be continuously oxidized back into NAD+ in order to keep this step going. If NAD+ is not available, the second half of glycolysis slows down or stops. If oxygen is available in the system, the NADH will be oxidized readily, though indirectly, and the high-energy electrons from the hydrogen released in this process will be used to produce ATP. In an environment without oxygen, an alternate pathway (fermentation) can provide the oxidation of NADH to NAD+.

Step 7. In the seventh step, catalyzed by phosphoglycerate kinase (an enzyme named for the reverse reaction), 1,3-bisphosphoglycerate donates a high-energy phosphate to ADP, forming one molecule of ATP. (This is an example of substrate-level phosphorylation. ) A carbonyl group on the 1,3-bisphosphoglycerate is oxidized to a carboxyl group, and 3-phosphoglycerate is formed.

Step 8. In the eighth step, the remaining phosphate group in 3-phosphoglycerate moves from the third carbon to the second carbon, producing 2-phosphoglycerate (an isomer of 3-phosphoglycerate). The enzyme catalyzing this step is a mutase (isomerase).

Step 9. Enolase catalyzes the ninth step. This enzyme causes 2-phosphoglycerate to lose water from its structure; this is a dehydration reaction, resulting in the formation of a double bond that increases the potential energy in the remaining phosphate bond and produces phosphoenolpyruvate (PEP).

Step 10. The last step in glycolysis is catalyzed by the enzyme pyruvate kinase (the enzyme in this case is named for the reverse reaction of pyruvate’s conversion into PEP) and results in the production of a second ATP molecule by substrate-level phosphorylation and the compound pyruvic acid (or its salt form, pyruvate). Many enzymes in enzymatic pathways are named for the reverse reactions since the enzyme can catalyze both forward and reverse reactions (these may have been described initially by the reverse reaction that takes place in vitro, under non-physiological conditions).

Outcomes of Glycolysis

One glucose molecule produces four ATP, two NADH, and two pyruvate molecules during glycolysis.

Learning Objectives

Describe the energy obtained from one molecule of glucose going through glycolysis

Key Takeaways

Key Points

  • Although four ATP molecules are produced in the second half, the net gain of glycolysis is only two ATP because two ATP molecules are used in the first half of glycolysis.
  • Enzymes that catalyze the reactions that produce ATP are rate-limiting steps of glycolysis and must be present in sufficient quantities for glycolysis to complete the production of four ATP, two NADH, and two pyruvate molecules for each glucose molecule that enters the pathway.
  • Red blood cells require glycolysis as their sole source of ATP in order to survive, because they do not have mitochondria.
  • Cancer cells and stem cells also use glycolysis as the main source of ATP (process known as aerobic glycolysis, or Warburg effect).

Key Terms

  • pyruvate: any salt or ester of pyruvic acid; the end product of glycolysis before entering the TCA cycle

Outcomes of Glycolysis

Glycolysis starts with one molecule of glucose and ends with two pyruvate (pyruvic acid) molecules, a total of four ATP molecules, and two molecules of NADH. Two ATP molecules were used in the first half of the pathway to prepare the six-carbon ring for cleavage, so the cell has a net gain of two ATP molecules and 2 NADH molecules for its use. If the cell cannot catabolize the pyruvate molecules further (via the citric acid cycle or Krebs cycle), it will harvest only two ATP molecules from one molecule of glucose.

image

Glycolysis produces 2 ATP, 2 NADH, and 2 pyruvate molecules: Glycolysis, or the aerobic catabolic breakdown of glucose, produces energy in the form of ATP, NADH, and pyruvate, which itself enters the citric acid cycle to produce more energy.

Mature mammalian red blood cells do not have mitochondria and are not capable of aerobic respiration, the process in which organisms convert energy in the presence of oxygen. Instead, glycolysis is their sole source of ATP. Therefore, if glycolysis is interrupted, the red blood cells lose their ability to maintain their sodium-potassium pumps, which require ATP to function, and eventually, they die. For example, since the second half of glycolysis (which produces the energy molecules) slows or stops in the absence of NAD+, when NAD+ is unavailable, red blood cells will be unable to produce a sufficient amount of ATP in order to survive.

Additionally, the last step in glycolysis will not occur if pyruvate kinase, the enzyme that catalyzes the formation of pyruvate, is not available in sufficient quantities. In this situation, the entire glycolysis pathway will continue to proceed, but only two ATP molecules will be made in the second half (instead of the usual four ATP molecules). Thus, pyruvate kinase is a rate-limiting enzyme for glycolysis.